Process of preparing solid propellant compositions using a dehydrating agent



ration of Ohio No Drawing. Filed July 10, 1961, Ser. No. 125,913

15 Claims. (Cl. 149-19) This invention relates to a new and improved method for forming solid propellant compositions. More specifically, this invention relates to an improved method for forming solid propellant compositions as disclosed in application Serial No. 109,409, filed August 9, 1949, now Patent No. 3,031,286, which compositions comprise an oxidizer dispersed in a resinous binder formed from a polyester component and a monomeric olefinic material.

In the formulation of solid propellants of the type shown in application Serial No. 109,409, it has been customary to employ a wetting agent such as lecithin during the casting operation. The Wetting agent in some way lubricates the ingredients present in the solid propellant so that the propellant mixture can be poured into a rocket casing and cast there to form a unitary mass.

Frequently difiiculties are encountered during the casting operation when the water content of the fuel components is too high. The excess water interacts in some way with the propellant formulation and forms a viscous mass which destroys the pourability of the propellant composition. As a result, the propellant composition cannot be cast properly because it will not flow readily into the rocket casing. When casting is attempted under these conditions, the material will not flow into the mold or an undue amount of air may be trapped within the propellant composition and thereby introduce voids within the cast propellant. This is detrimental because the voids increase the surface area of the cast propellant and thereby increase its burning rate. This can result in explosion of the rocket during firing. Further, the presence of voids within the cast propellant is detrimental because it produces an uneven burning rate throughout the propellant charge unless the void'disu'ibution is completely uniform. For example, if the percentage of void distribution is greater in one portion of the rocket fuel than in another, the first portion will burn at a higher rate and thereby introduce stresses within the rocket charge. This can result in cracking of the charge and an explosion during firing.

An object of my invention is to provide a method for casting solid rocket charges which solves the problems of the prior art as outlined above. A further object of my invention is to provide a novel method in which a dehydrating agent is added to the solid propellant charge prior to casting, which dehydrating agent also acts as a Wetting agent in improving the castability of the solid propellant mixture. Further objects will become apparent from the specification and claims which follow.

My invention involves addition of a dehydrating agent as described below, which can be, for example, a metal hydride such as magnesium hydride or a metal carbide such as alkaline earth metal carbides, i.e., calcium carbide, to the propellant mixture, as defined in application Serial No. 109,409, prior to the casting of the solid fuel composition. A preferred dehydrating agent for use in my inventive process is calcium carbide. I have found United States Patent "ice Patented Apr. 28, 1964 the pourability of the solid ingredients in the propellant formulation; it must not leave a deleterious residue; and it must be a material which is essentially nonreactive with the various components in the solid fuel formulation such as the resin binder and the oxidizer. If the dehydrating agent does not meet these requirements, it can produce imperfections in the cast propellant which can cause uneven burning and explosion when the rocket is ignited. I have found that calcium carbide meets all of the above requirements for a dehydrating agent having application in my process.

Generally, I employ a small quantity of dehydrating agent which ranges from a trace to about 1 percent by weight of the mixed fuel formulation. Greater amounts of the dehydrating agent can be employed. In general, however, this is not necessary since the quantity of water usually present in the fuel ingredients can be removed with this quantity of dehydrating agent.

Following the addition of the dehydrating agent it is desirable in some cases to warm the fuel mixture so as to heat the gases which are formed from the reaction of the dehydrating agent with water. This aids in the escape of the gases from the fuel formulation prior to castmg.

I can add the dehydrating agent directly to the polyester binder prior to its admixture with the oxidizer and curing catalyst or I can add the dehydrating agent to the formulated propellant composition including the resin binder, oxidizer, and curing catalyst. As stated in lines 16 et seq., page 7, in applicatiou'Serial No. 109,409, a polymerization or curing catalyst is useful in speeding the rate of polymerization of the polyester resinmonomeric olefin mixture. The catalyst is customarily added at a selected time during the fuel compounding operation such that polymerization of the binder will not occur until after the propellant composition has been cast. In practice the precise time of adding the curing catalyst is within the skill of the operator who will add the curing catalyst when he desires to start the polymerization of the binder. Thus, for example, if the curing catalyst is such that it will cause the binder to polymerize in 45 minutes, the operator will not add the catalyst if another hour of mixing is required to uniformly mix the fuel components. If, on the other hand, only an additional 15 minutes of mixing is required before casting, the operator could add the curing catalyst so as to initiate polymerization during the mixing operation.

Generally, the dehydrating agent is added prior to the addition of the curing catalyst. This is not a fixed rule and can be altered by the operator. The important thing is, however, that the propellant composition be sufiiciently fluid after addition of the dehydrating agent to-allow for escape of the gases formed by the reaction of the dehydrating agent with the Water present in the fuel formulation.

Although my invention relates to a method for forming propellant compositions of the'type described in application Serial No. 109,409 and not to the compositions themselves, I will, in this application, summarize the propellant composition described in the earlier application. This will serve to incorporate by reference the disclosure of the prior application and will also serve to describe the environment in which my inventive process is employed.

Specifically, the fuel formulations produced by my method comprise a finely divided oxidizer which is uniformly distributed in a resinous binder which acts as a fuel. The resin generally'comprises a polyester component, which is the reaction product of a polycarboxylic acid and a polyhydric alcohol, into which there is in- The polyester component should possess some degree of unsaturation in the molecule in order to permit it to heteropolymerize with the olefinic component which also possesses unsaturation. In general, any olefin which is compatible with the resin and which will polymerize with it is suitable. This includes essentially all unsubstituted olefins and in addition many substituted olefins. The reason for having unsaturation present in the polyester is to permit the unsaturated polyester to polymerize with a double bond in the vinyl, allyl, or other olefinic component. When a sufficient amount of cross-linkage occurs, the resin becomes thermosetting. With a lesser degree of cross-linkage the resin will be thermoplastic and in some cases the resin may possess some properties of both a thermoplastic and a thermosetting resin.

The polyester component is, in general, prepared by reacting a dihydric or polyhydric alcohol with a polycarboxylic acid in the presence of a monomeric vinyl, allyl, or other olefinic component. The unsaturation required to permit the polyester to heteropolymerize with the monomeric olefinic component may be supplied by employing an unsaturated polyhydric alcohol or an unsaturated polycarboxylic acid. The percentage of the unsaturated acid or anhydride should be sufiicient to permit the necessary amount of copolymerization between the vinyl, allyl, or other olefinic component and the polyester. The polyester may be present in an amount varying between about 10 and '70 percent by weight based upon the weight of the polyester-olefinic additive mixture. In general, however, a mixture comprising 50 percent of the polyester to 50 percent by weight of the olefinic additive produces a satisfactory polyester-resin type of matrix for the propellant.

The solid propellant comprises a uniform dispersion of a finely divided inorganic oxidizer in a polyester resin matrix of the type described above. The resin acts as a binder and, regardless of substituents in the molecule, will serve as a fuel if the propellant contains a sufficient amount of oxidizer to insure the necessary oxidation of the organic material; usually all of the carbon is oxidized to CO and one-third of the hydrogen is oxidized to water. As stated above, the preferred condensation product is obtained by reacting a polyhydric alcohol with a polycarboxylic acid having a predetermined degree of unsaturation in the molecule. An especially useful form of this condensation product is the reaction product of sebacic acid and a polyhydric alcohol such as propylene glycol to which there has been added a small amount of unsaturated anhydride such as maleic anhydride. This product is commonly called a modified alkyd resin.

The percentage of unsaturation in the polyhydric alcohol-polycarboxylic acid mixture used to form the polyester should be between about 2 and 100 percent by weight based on the weight of the total polycarboxylic acid or polycarboxylic acid mixture. The preferred percentage of unsaturation is between about 10 and percent by weight based on the weight of the acidic component. The polyhydric alcohol and polycarboxylic acid will react in stoichiometric proportions. Nevertheless, it is usually a better practice to use an excess of the polyhydric alcohol beyond the stoichiometric amount. The excess alcohol is then removed from the finished polyester in order to make the resulting product substantially free from unnecessary matter or impurities.

One particular form of heteropolymerizable mixture which I prefer comprises 4 moles of sebacic acid, 1 mole of maleic anhydride, and 5 moles of propylene glycol. Such an alkyd resin is available commercially and is hereafter referred to as Resin A. A related product, also available commercially and hereafter referred to as Resin B, already contains the olefinic component, namely styrene. It is a heteropolymerizable resin compounded by mixing approximately 50 percent by weight of styrene with 50 percent by weight of Resin A.

Another example of such a resin, which has been found to be particularly useful, is the condensation prodnot of adipic acid and diethylene glycol to which there has been added a small amount of maleic anhydride. Such a resin, hereafter referred to as Resin D, is made by mixing 7 moles of adipic acid, 3 moles of maleic anhydride, and 11 moles of diethylene glycol. A related resin, hereafter referred to as Resin C, is made by mixing 7 moles of adipic acid, 3 moles of maleic anhydride, and 12 moles of diethylene glycol. Other resins, compounded by using other polyhydric alcohols, polycarboxylic acids, and anhydrides, may also be used.

The specific resins identified above as Resin A, Resin C, and Resin D can be made to polymerize with the vinyl, allyl, or other olefinic type of monomers to form the desired heteropolymerized resin. The amount of olefinic monomer such as, for example, styrene may range from about 25 percent to about 100 percent by weight based on the weight of the monomer-resin mixture and the amount of the monomer to be used in each case is determined by the particular properties which are desired in the finished resin. The olefinic monomers listed above are all liquids and thereby serve as solvents for the heavier alkyd resin, thus facilitating the ease with which the oxidizer may be dispersed throughout the liquid resin before curing.

For the oxidizer, I prefer to use any stable, solid, inorganic oxidizer. The oxidizer is a substance which may be incorporated in the polyester resin-unsaturated polycarboxylic-olefinic monomer mixture by stirring and mixing and I preferably add the oxidizer to the mixture while the resin is in its liquid state. Examples of suitable oxidizers are the inorganic substances including the chromates, dichromates, permanganates, nitrates, chlorates, and perchlorates, such as the alkali metal salts of these radicals including sodium, potassium, lithium, rubidium, and caesium, and also the nonmetallic salts of the same radicals such as ammonium and hydrazine. The selection of the oxidizing material depends upon the type of propellant and the specific burning properties desired. The preferred oxidizers are the perchlorates, especially the perchlorates of potassium and ammonium. The amount of oxidizer added to the resinous mixture is usually between 45 percent and percent by weight of the total propellant composition and the weight of the polyester resin-unsaturated polycarboxylic-olefinic monomer mixture is between 55 percent and 10 percent of the same propellant composition.

Catalytic substances are particularly useful for speeding up the rate of polymerization of the said polyester resin-monomer mixtures with the oxidizer added. Such catalytic substances are the organic peroxides and the organic peresters. The temperature used for curing is dependent somewhat upon the nature of the catalyst and the time during which it is desired to accomplish complete polymerization.

Some catalysts such as l-hydroxy cyclohexyl hydroperoxide and cumene hydroperoxide are capable of polymerizing certain resins such as Resin B at room temperature if the charge is permitted to cure for a sufficiently long period of time.

The organic peroxides or peresters should preferably be soluble or compatible with the polyester resin. However, in some instances even an insoluble organic peroxide or perester functions as a catalyst as long as it can be made to decompose and liberate a free oxygen radical. Specific examples of compounds which are suitable catalysts for this polymerization reaction are tertiary butyl hydroperoxide, curnene hydroperoxide, benzoyl peroxide, lauryl peroxide, acetobenzoyl peroxide, ditertiary butyl peroxide, methyl ethyl ketone peroxide, lhydroxy cyclohexyl hydroperoxide, cyclo alkane hydrocarbon peroxide, and other hydroperoxides WhlCh are not too volatile at the curing temperature.

Specific examples of suitable peresters are tertiary butyl perbenzoate and ditertiary butyl diperphthalate.

Such catalysts should be present in the polyester resincarboxylic olefinic mixture during the time it is subjected to the curing process. In general the weight of the catalyst employed to bring about this result is approximately 0.5 percent by weight based on the weight of thecombined polyester resin-"carboxylic olefinic mixture. If desired, larger amounts of the catalysts may be employed than those indicated. V

border to provide steady burning at low pressure with some oxidizers, it is beneficial to incorporate into the propellant mass approximately 1 percent by weight of carbon black which is added to the liquid mixture at the time the'oxidizer isincorporated'therein and before the mixture is cured. I

The manner in which my method is practiced is generally, as set forth above, to add a dehydrating agent to either a liquid polyester resin-olefinic monomer mixture or to such a mixture containing also the oxidizer. After the addition of the dehydrating agent (of the nature described previously) the mixture is stirred until it has attained a uniform consistency.

If the dehydrating agent has been added only to the polyester resin-olefinic monomer mixture, the oxidizer is added subsequently and the mixture is stirred until a uniform consistency has been obtained. Generally, the stirring is done at room temperature until all of the oxidizer has been added and the mixture has 'a uniform consistency.

At the appropriate time during the mixing operation (as determined by the operator), a polymerization catalyst is added. The catalyst is generally added at or before the addition of the oxidizer. The stirred mixture, including the polymerization catalyst, is then cast into a suitable mold, ordinarily cylindrical in outline, and the material is allowed to cure. The cast mass is generally cured at temperatures ranging from ambient to 220 F. If lower temperatures are used the curing time is relatively long, while .at higher temperatures the curing time is relatively short. If, for any reason, it is desired by the operator to lengthen the curing time, a curing rate retardant can be added to the propellant mixture.

To further illustrate the scope of my invention there are set forth the following examples in which all parts and percentages are by weight unless otherwise indicated.

Example I A propellant formulation was made up which contained 9-8.4 parts of a resin which was about 50 percent by weight of styrene and 50 percent by weight of Resin D; 1.2 parts of a percent solution of lecithin i-n styrene; and 0.40 part of methyl amyl ketoperoxide. After mixing these components for about 6 minutes there was then added 300 parts of ammonium perchlorate. Mixing was continued until the ammonium perchlorate was uniformly dispersed and the formulation was then poured into a container maintained at 70 F. After about 5 /2 hours, the propellant mixture was observed to have gelled. Gelation was determined by means of a standard test which involved inserting a 3 mm. diameter clean glass probe into a sample of the propellant mixture contained in an eight-dram vial having an outside diameter of 25 mm. and a height of 95 mm. The propellant sample size was about 5 drams and occupied about two-thirds of the volume of the vial.

At twenty-minute intervals the probe was raised and lowered within the glass vial to determine whether the propellant mixture had gelled. When the rod became imbedded in the propellant mixture so that it could not be moved, gelation was considered to have occurred.

When Example I was repeated without the wetting agent (in this case lecithin) I was unable to obtain a castable propellant. The propellant formulation did not flow properly because of the fact that its viscosity was too high.

Example 11 Example I was substantially repeated by mixing 97.2 parts of a resin comprising about 50 percent styrene and about 50 percent of Resin D with-about 2 parts of calcium carbide and about 0.4 part of a '1 percent solution of tertiary butyl cateclrol in styrene. After these ingredients had been thoroughly mixed, there was added about 0.40 part of ethyl'amyl'ketoperoxide which acted as the curing catalyst; After mixing for an additional 5 minutes, about 300 parts of ammonium perchlorate (oxidizer) were added and mixed into the propellant formulation. The material was then cast into a container which'was maintained a't'about vF. After 12- hours and 12 minutes, the propellant composition was'found to have gelled. Gelation was determined using the test methodset forth'in Example I.

'I he tertiary butyl' catechol employed in Example II acted as' a curing rate retardant. This helped to'loffset the increase in the gelling rate caused from addition of the calcium carbide and elimination of the wetting agent.

When my method is applied to other propellant compositions as shown inExamples 1 through 12 ofapplicanon Serial No. 109,409, similar results are obtained. That is to say, that in each case a readily castable, essentially dry propellant fiormulation is obtained.

On testing of propellants formed by my method, it was found that their burning rate was greater than that of propellants formed by conventional methods, i.e., containing some water and using a wetting agent. To illustrate, cure-d propellant compositions corresponding to Examples I and II were subjected to a standard Crawford- Bomb burning rate test. In this test, confined propellant strands (coated with a low burning acrylate polymer) are burned under controlled nitrogen pressure. The burning rate is determined by placing low melting wires within the propellant strands at right angles to the longitudinal axis of the strand. The wires are connected to a timing device which records the time that the wires are melted through burning of the strand.

When so tested, the propellant of Example I had a burning rate of 0.205 inch/ second at a nitrogen pressure of 600 p.s.i. and a burning rate of 0.28 inch/second at 1000 p.s.i. The propellant of Example II had a burning rate of 0.25 inch/ second at 600 p.s.i. and 0.38 inch/ second at 1000 p.s.i. The burning rate of the propellant of Example II represents about a 20 percent improvement over that of Example I at 600 p.s.i. and about a 35 percent improvement at 1000 p.s.i.

Since the propellant compositions of Examples I and II are essentially the same, the improvement in the burn ing rate of that of Example II can be directly attributed to my method.

When dehydrating agents other than calcium carbide such as metal hydrides and alkaline earth metal carbides are used in my process, similar results are obtained.

The compositions produced by my method have demonstrated utility as solid propellants.

Having fully defined my novel method and its use in producing superior fuel formulations, I desire to be limited only by the scope of the appended claims.

I claim:

1. Process comprising mixing a solid propellant composition composed of an inorganic oxidizing salt, a polyester resin formed from the condensation of a saturated polyhydric alcohol and a polycarboxylic acid, and a monomeric olefinic compound, in the presence of an effective quantity of a dehydrating agent capable of reacting with any water present in the reaction mixture to form a gas, said dehydrating agent being selected from the group consisting of metal hydrides and metal carbides.

2. The process of claim 1 in which there is also present in the reaction mixture a polymerization curing catalyst.

3. The process of claim 2 wherein said polymerization curing catalyst is added after the addition of said dehydrating agent. i

4. The process of claim 1 wherein said dehydrating agent is calcium carbide.

5. The process of claim 1 wherein the propellant composition is heated to expel gas formed from the reaction of the dehydrating agent and water.

6. The process of claim 1 including the additional step of casting the mixed propellant into a mold.

7. The process Comprising mixing a solid propellant composition composed of an inorganic oxidizing salt, a polyester resin formed from the condensation of a saturated polyhydric alcohol and a polycarboxylic acid, and a monomeric olefinic compound, said polyester resin being present in an amount ranging from about to about 70 percent by weight of the combined weight of the polyester resin and monomeric olefinic compound, in the presence of an effective quantity of a dehydrating agent capable of reacting with any water present in the reaction mixture to form a gas, said dehydrating agent being selected from the group consisting of metal hydrides and metal carbides.

8. The process of claim 7 wherein said polyester resin constitutes about 50 percent by weight of the polyester resin-monomeric olefinic compound mixture.

9. The process of claim 7 wherein said inorganic oxidizing salt constitutes between about to about percent by weight of the total propellent composition.

10. The process of claim 9 wherein the monomeric olefinic compound is styrene and the polyester resin is formed by mixing about 7 moles of adipic acid, 3 moles of maleic anhydride, and 11 moles of diethylene glycol.

11. The process comprising mixing a solid propellant composition composed of an inorganic oxidizing salt, a polyester resin formed from the condensation of a saturated polyhydric alcohol and a polycarboxylic acid, and a monomeric olefinic compound, in the presence of calcium carbide in an amount ranging from a trace to about one percent by weight of the total propellant formulation.

12. The process of claim 1 wherein said dehydrating agent isv an alkaline earth metal carbide.

13. The process of claim 7 wherein said dehydrating agent is an alkaline earth metal carbide.

14. The process of claim 13 wherein said alkaline earth metal carbide is calcium carbide.

15. The process of claim 14 wherein said calcium carbide is present in an amount ranging from a trace to about one percent by weight of the total propellant formulation.

No references cited. 

1. PROCESS COMPRISING MIXING A SOLID PROPELLANT COMPOSITION COMPOSED OF AN INORGANIC OXIDIZING SALT, A POLYESTER RESIN FORMED FROM THE CONDENSATION OF A SATURATED POLYHYDRIC ALCOHOL AND A POLYCARBOXYLIC ACID, AND A MONOMERIC OLEFINIC COMPOUND, IN THE PRESENCE OF AN EFFETIVE QUANTITY OF A DEHYDRATING AGENT CAPABLE OF REACTING WITH ANY WATER PRESENT IN THE REACTION MIXTURE TO FORM A GAS, SAID DEHYDRATING AGENT BEING SELECTED FROM THE GROUP CONSISTING OF METAL HYDRIDES AND METAL CARBIDES. 